Force-sensitive occupancy sensing technology

ABSTRACT

A force-sensitive capacitive sensor that includes a first conductive plate, a second conductive plate that is spaced apart from the first conductive plate, and a compressible dielectric insulator positioned between the first conductive plate and the second conductive plate. The sensor also includes a first protective insulator, a second protective insulator sealed to the first protective insulator to encase the first conductive plate, the second conductive plate, and the compressible dielectric insulator, and a circuit attached via wires to the first conductive plate and the second conductive plate. The sensor may also include electromagnetic shielding. The circuit is configured to sense a change in capacitance between the first conductive plate and the second conductive plate caused by compression of the compressible dielectric insulator resulting from a person occupying the sensor or a support surface positioned above the sensor, and transmit output based on the sensed change in capacitance.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/828,330, filed Mar. 14, 2013, now allowed, which claims thebenefit of U.S. Provisional Application No. 61/625,237, filed Apr. 17,2012. The prior applications are incorporated herein by reference intheir entirety for all purposes.

TECHNICAL FIELD

This disclosure relates to capacitive occupancy sensing technology.

BACKGROUND

Recent technological advancements have facilitated the detection ofoccupancy on human support surfaces such as beds, cushioned seats, andnon-cushioned seats (e.g., chairs and sofas) via sensors placed directlyabove or below the support surface (e.g., cushion or mattress). Morespecifically, a binary occupancy sensor produces a distinct output whena support surface is either occupied or unoccupied. Beyond supportsurface detection, a broad application space exists for human-centricbinary occupancy sensing, ranging from safety to wellness assessment.For example, bed and seat occupancy sensors can be utilized to measureand assess sedentary behavior (e.g., time spent in bed or seat) and fallrisk (e.g., bed entries and exits, time spent away from bed, etc.).Occupancy can be measured with electrically conductive contacts (e.g.,electrical contact created when occupied) or more complex sensingmechanics (e.g., resistive, load cell, pressure, etc.) filtered toproduce binary output.

More complex sensing elements can also measure small variations in forceapplied to support surfaces and provide corresponding variable output.Such sensors are typically placed above the support surface and indirect contact with the sensed body. Combined with sophisticated signalfiltering and processing, diverse applications of such force-sensitivesensors range from sleep quality measurement to detection of breathingrate and sleep apnea.

SUMMARY

Techniques are described for capacitive occupancy sensing.

Implementations of the described techniques may include hardware, amethod or process implemented at least partially in hardware, or acomputer-readable storage medium encoded with executable instructionsthat, when executed by a processor, perform operations.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1 and 3 illustrate an example capacitive sensor.

FIG. 2 is a diagram that illustrates an example circuit.

FIG. 4 is a flow chart illustrating an example process.

FIG. 5 illustrates an example signal.

DETAILED DESCRIPTION

Techniques are described for multi-modal, capacitive, force-sensitivebed and seat sensing. The sensor's flexible form allows it to beutilized both above support surfaces (e.g., in direct contact with thesensed body) and below support surfaces (e.g., below a mattress orcushion). Furthermore, the sensor's capacitive sensing element, combinedwith in-sensor computational processing processes, allow for both binaryoccupancy detection and high precision, force-sensitive variablemeasurement.

FIG. 1 illustrates an example capacitive sensor 100. The capacitivesensor 100 is a multi-modal capacitive sensor that includes conductiveplates 110 and 120 (made either rigid or flexible by material selectionand/or supportive backing). A compressible dielectric insulator 130 ispositioned between the conductive plate 110 and the conductive plate120. A top protective insulator 140 and a bottom protective insulator150 are sealed together to encase the components of the capacitivesensor 100. The top protective insulator 140 and the bottom protectiveinsulator 150 may include anti-microbial or non-slip fabrics andsurfaces. The conductive plates 110 and 120 may be constructed ofmetalized foil with optional plastic backing to enhance rigidity. Thecapacitive sensor 100 also may include electromagnetic shielding, whichmay include metal surfaces or metal plates embedded into theconstruction of the capacitive sensor 100. For example, electromagneticshielding may be placed between the top protective insulator 140 and thebottom protective insulator 150. The compressible dielectric insulator130 may be constructed of non-conductive foam. The capacitive sensor 100also includes a computational circuit 160 attached via wires to theconductive plates 110 and 120.

The two conductive plates 110 and 120 are placed on either side of thecompressible dielectric insulator 130 to construct a capacitive elementinfluenced by external applied force. When external force (e.g., aperson's weight) is applied to the sensor 100, the dielectric insulator130 compresses, the distance between the conductive plates 110 and 120is reduced, and the capacitance is increased. This relationship isillustrated below in Equation 1, where C is the capacitance, ∈ is thepermittivity of the insulating dielectric 130, A is the area of theconductive plates 110 and 120, and d is the distance between theconductive plates 110 and 120. Provided all layers in the capacitivesensing element are flexible, the sensor 100 may be placed either aboveor below a cushioned support surface to detect occupancy or smallvariations in applied force.

$\begin{matrix}{{{Capacitance}\mspace{14mu}{Relationship}}{C = \frac{ɛ\; A}{d}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

FIG. 2 illustrates an example of the circuit 160. As shown in FIG. 2,the circuit 160 includes digital and analog components that convertcapacitance to a digital value. For instance, the circuit 160 includes afirst sensing element input 210 that is connected to the conductiveplate 110 and a second sensing element input 220 that is connected tothe conductive plate 120. The circuit 160 also includes a pre-processingcircuit 230 and a processor 240. The pre-processing circuit 230 receivesinput from the first sensing element input 210 and from the secondsensing element input 220 and performs pre-processing on the receivedinputs. The pre-processing circuit 230 provides results ofpre-processing to the processor 240. The pre-processing circuit 230 andprocessor 240 digitally process the signals for the conductive plates todetect occupancy or quantify small changes in externally applied force.For example, the pre-processing circuit 230 may convert sensedcapacitance between the conductive plates 110 and 120 into anoscillating signal of varying frequency at digital logic levels.

Moreover, the circuit 160 may include a wireless radio 250 thattransmits capacitance, occupancy, or measured externally applied forcedata to a remote location (e.g., a base station, a mobile device, awireless router, etc.). The circuit 160 also may include localmemory/storage 260 that stores capacitance, occupancy, or measuredexternally applied force data. The memory/storage 260 may temporarilystore capacitance, occupancy, or measured externally applied force dataprior to transmission by the wireless radio 250 to a remote location(e.g., a base station, a mobile device, a wireless router, etc.).Further, the circuit 160 may include input/output and user interfacecomponents 270 (e.g., a button and a light-emitting diode (LED)) tofacilitate user interaction. Because the sensing element may bepositioned below a cushioned support surface of unknown weight, userinteraction may be used for sensor calibration or taring. For example,the circuit 160 may receive user input that initiates a calibrationprocess and that indicates that no user-applied force is being providedto the sensing element. In this example, the circuit 160 may measure theweight in the unoccupied state based on receiving the user input and usethe measured weight to calibrate the sensor. Calibration may promotehigher accuracy measurement of external forces, such as human supportsurface loading.

To measure capacitance, the circuit 160 may employ various processes.For example, the circuit 160 may utilize a Schmitt-trigger along with aresistor to oscillate between digital logic levels (“0” and “1”) at afrequency directly related to the sensed capacitance and the “RC timeconstant” created with the added resistance. In this example, theoscillating signal serves as a clock source for a counter. Thedifference in counter value is measured over a known period of time(obtained from another time source), and the number in the counterdirectly corresponds to the sensed capacitance.

In another example, the circuit 160 measures capacitance by introducinga transient input in voltage and/or current and then measuring theresponse to the transient input with respect to time. In this example,the circuit 160 calculates capacitance based on the measured responseand time. These processes, among others, may be used to sense minutechanges in capacitance with small, inexpensive, and power efficientcircuitry. The power efficiency may allow the circuit 160 to beexternally or battery powered.

FIG. 3 illustrates an example implementation of the capacitive sensor100 with both top and bottom views being shown. As shown in the topview, the top protective insulator 140 defines an external top surfaceof the capacitive sensor 100. As shown in the bottom view, the bottomprotective insulator 150 defines an external bottom surface of thecapacitive sensor 100. The circuit 160 is positioned within a circuitbox, which is exposed. Wires connect the circuit 160 to the conductiveplate 110 and the conductive plate 120, which are positioned between andcovered by the top protective insulator 140 and the bottom protectiveinsulator 150.

FIG. 4 illustrates an example process 400 for occupancy sensing. Theoperations of the example process 400 are described generally as beingperformed by the circuit 160. In some implementations, operations of theexample process 400 may be performed by one or more processors includedin one or more electronic devices. As shown in FIG. 4, the circuit 160provides computational capabilities to calibrate or tare the sensor(410), calculate capacitance (420), determine occupancy state or otherforce-sensitive measures (430), determine operational state (440), cacheor store data (450), and transmit data off of the sensor (e.g.,wirelessly) (460).

The circuit 160 calibrates or tares the sensor 100 (410). The sensor 100may be tared manually or automatically. To tare the sensor 100 manually,the circuit 160 determines that the sensor 100 is unoccupied based onreceiving user input (e.g., a press of a button on the computationalcircuit device) or based on receiving, from another electronic device, asignal that initiates a calibration process (e.g., a wirelessly receivedcommand). Upon initiation of the calibration process, the circuit 160determines a capacitance measured by the circuit 160 in the unoccupiedstate and uses the determined capacitance as a baseline measurement forcalibrating the sensor 100. Sensor calibration may be performedperiodically, as the unoccupied capacitance value may change over time(e.g., due to small differences in load distribution andmaterial-induced hysteresis of both the cushioned support surface andthe compressible insulator between the conductive plates). The circuit160 may automatically perform periodic calibration without requiringuser input or an outside signal to initiate the calibration.

Other processes of automatic calibration or taring also may be employedwithout the use of manual or command-initiated device input. Forexample, capacitance values may be statistically profiled andunsupervised machine learning processes may be implemented to classifyoccupancy state.

After the sensor 100 has been calibrated, the circuit 160 calculatescapacitance (420). To measure capacitance, the circuit 160 may employvarious processes. For example, the circuit 160 may utilize aSchmitt-trigger along with a resistor to oscillate between digital logiclevels (“0” and “1”) at a frequency directly related to the sensedcapacitance and the “RC time constant” created with the addedresistance. In this example, the oscillating signal serves as a clocksource for a counter. The difference in counter value is measured over aknown period of time (obtained from another time source), and the numberin the counter directly corresponds to the sensed capacitance.

In another example, the circuit 160 measures capacitance by introducinga transient input in voltage and/or current and then measuring theresponse to the transient input with respect to time. In this example,the circuit 160 calculates capacitance based on the measured responseand time.

The circuit 160 may calculate a change in capacitance by computing adifference between the measured capacitance and the baseline capacitancemeasured during calibration. The circuit 160 may use the change incapacitance to measure the force applied to the sensor 100 by a user.

After the circuit 160 calculates capacitance, the circuit 160 determinesan occupancy state and other force-sensitive measurements (430). Forinstance, the circuit 160 may determine a binary occupancy state (e.g.,occupied or not occupied) based on the calculated capacitance and alsomay determine high precision, force-sensitive measurements based on thecalculated capacitance. The circuit 160 may determine the highprecision, force-sensitive measurements by translating the calculatedcapacitance to force applied to the sensor 100. The force may becalculated by determining a force-capacitance equivalence function atdesign time or at build time. The function may be determined by applyingexternal loads of known forces to the sensor 100 and measuring thesensor 100 output capacitance. The force-capacitance equivalencefunction may be represented as an interpolation of the known loads andmeasured capacitances. The circuit 160 may calculate the force usingthis equivalence function or estimate the force using a capacitance toforce lookup table stored on the processor 240 or in storage 260.

The circuit 160 may use various processes to determine the binaryoccupancy state. For instance, the circuit 160 may detect occupancybased on measuring a force greater than a threshold and detect a lack ofoccupancy based on measuring a force less than the threshold. To reducefalse activations or deactivations, a process may be deployed on theembedded circuit 160 (e.g., on a processor or microcontroller) to managesmall variations. The process is explained below with respect to FIG. 5.

FIG. 5 illustrates an example signal corresponding to a transition to,and back from, an occupied state with notations for relevant variables.Sampled values corresponding to the unoccupied state are averaged over anumber of samples (denoted as T) to subtract small changes in appliedforce and noise from both the electrical and mechanical systems.Therefore, at any time t, the process has an estimate of the averageacquired signal in the unoccupied state (U_(AVG)). Another value & isdefined as the amount of signal change required for activation. Anactivation threshold A is set as described in Equation 2. Consequently,the activation threshold is updated on the time interval of T tocompensate for small changes in force, and therefore sensed capacitance,corresponding to the unoccupied state.A=U _(AVG)−∈   Equation 2: Activation Threshold Equation

Once the signal crosses the activation threshold for a pre-definednumber of samples, sampled values corresponding to the occupied stateare averaged over T samples to continually update the “occupied signalaverage” (O_(AvG)), and U_(AVG) is no longer calculated while in theoccupied state. Additionally, for each update of O_(AVG), the differencebetween the most recent U_(AVG) and O_(AVG) is calculated and stored ina variable denoted as δ. Similar to the activation threshold, adeactivation threshold (D) is set. Unlike the activation threshold,however, this new threshold is set relative to the recovering proportion(α) expressed as a number between 0 and 1. This relationship isexpressed in Equation 3. After the signal crosses the deactivationthreshold for a pre-defined number of samples, operation resumes aspreviously described for the unoccupied state.D=O _(AVG)+δ·α   Equation 3: Activation Threshold Equation

This process to set the activation and deactivation thresholds, combinedwith the ability to manually set U_(AVG) using a button interface orother tare initiation mechanism, allows the sensor system to detectoccupancy in a multitude of scenarios with unknown support surface andsubject weights. In the unoccupied state, the circuit 160 compares themeasured capacitance to the activation threshold and determines that thesensor 100 is occupied based on the comparison revealing that themeasured capacitance meets the activation threshold. Based on thedetermination that the sensor 100 is occupied, the circuit 160 moves tothe occupied state and begins comparing measured capacitance to thedeactivation threshold. Based on the comparison revealing that themeasured capacitance exceeds the deactivation threshold, the circuit 160determines that the sensor 100 is not occupied. Based on thedetermination that the sensor 100 is not occupied, the circuit 160returns to the unoccupied state and resumes comparing measuredcapacitance to the activation threshold. The process continues in thismanner as the circuit 160 detects occupancy and lack of occupancy.

In some implementations, in addition to binary occupancy state,different force measurements may be determined at different locationsacross the sensor 100. In these implementations, the sensor 100 iscapable of localizing force measurements to specific regions of thesensor surface. For instance, the sensor 100 may include multiplesensing outputs provided for each of the conductors. Each of themultiple sensing outputs may be associated with a specific region of thesensor surface and may be analyzed to provide a force (e.g.,capacitance) measurement for the specific region.

In other examples, the conductors may be divided into multiple, separateplates across the sensor surface. In these examples, a force (e.g.,capacitance) measurement may be taken for each pair of separate platesand, therefore, different force measurements may be determined atdifferent locations across the sensor 100.

The different force measurements may be used to determine a distributionof force across the sensor 100. The distribution of force across thesensor 100 may provide additional data as compared to occupancy statealone and may be analyzed to detect various conditions. For instance,the distribution of force may be analyzed to assess risk for pressureulcer when occupying the sensor 100. In addition, the distribution offorce may be analyzed for changes to determine whether a person isrestless when occupying the sensor 100 (e.g., restless during sleep),even though the sensor 100 remains occupied. The sensor 100 could alsobe used to measure forces exerted by multiple individuals occupying thesame mattress.

The circuit 160 determines operational state of various components ofthe sensor 100 (440). For instance, the circuit 160 may determine abattery state of the circuit 160 battery. The circuit 160 also maydetect when various trouble conditions arise within the sensor 100(e.g., a connection to a conductive plate of the sensor is lost). Thecircuit 160 may determine any measurable operational state of any of thecomponents of the sensor 100 or circuit 160 and use the one or moremeasured operational states to proactively address any detected troubleconditions or to attempt prevention of trouble conditions before theyarise.

The circuit 160 caches or stores data 100 (450). For instance, thecircuit 160 may store values related to the taring or calibrationprocess, in addition to state variables describing the sensor'soperation state (e.g., battery state, trouble conditions, etc.). Thecircuit 160 also may store measured capacitance values, determinedoccupancy states, and/or other force measurements. The circuit 160 maystore any data measured or determined by the circuit 160. The storagemay be temporary and deleted after the data is transmitted to anexternal device.

The circuit 160 outputs data from the sensor 100 (460). For example, thecircuit 160 may communicate to a user or transmit to an external devicevalues related to the taring or calibration process, in addition tostate variables describing the sensor's or circuit's operation state(e.g., battery state, trouble conditions, etc.). The circuit 160 alsomay communicate to a user or transmit to an external device measuredcapacitance values, determined occupancy states, and/or other forcemeasurements. The circuit 160 may continuously or periodically transmitdata collected by the circuit 160. In some examples, the circuit 160 maydelay transmission until the storage on the circuit 160 is nearly full(e.g., within a threshold storage amount of being full) and thentransmit all of the stored data. In addition, the circuit 160 maytransmit data upon request or may have rules that define when datashould be transmitted based on the values measured. For instance, thecircuit 160 may transmit data to indicate a measured force above athreshold value, a determined change in occupancy state, or a particularoccupancy state that lasts more than a threshold period of time. Anyrules may be set to determine when the circuit 160 transmits data andwhat data the circuit 160 transmits. For example, the circuit 160 maypredict occupancy states and only transmit measured occupancy statesthat differ from predicted states.

The described systems, methods, and techniques may be implemented indigital electronic circuitry, computer hardware, firmware, software, orin combinations of these elements. Apparatus implementing thesetechniques may include appropriate input and output devices, a computerprocessor, and a computer program product tangibly embodied in amachine-readable storage device for execution by a programmableprocessor. A process implementing these techniques may be performed by aprogrammable processor executing a program of instructions to performdesired functions by operating on input data and generating appropriateoutput. The techniques may be implemented in one or more computerprograms that are executable on a programmable system including at leastone programmable processor coupled to receive data and instructionsfrom, and to transmit data and instructions to, a data storage system,at least one input device, and at least one output device. Each computerprogram may be implemented in a high-level procedural or object-orientedprogramming language, or in assembly or machine language if desired; andin any case, the language may be a compiled or interpreted language.Suitable processors include, by way of example, both general and specialpurpose microprocessors. Generally, a processor will receiveinstructions and data from a read-only memory and/or a random accessmemory. Storage devices suitable for tangibly embodying computer programinstructions and data include all forms of non-volatile memory,including by way of example semiconductor memory devices, such asErasable Programmable Read-Only Memory (EPROM), Electrically ErasableProgrammable Read-Only Memory (EEPROM), and flash memory devices;magnetic disks such as internal hard disks and removable disks;magneto-optical disks; and Compact Disc Read-Only Memory (CD-ROM). Anyof the foregoing may be supplemented by, or incorporated in,specially-designed ASICs (application-specific integrated circuits).

It will be understood that various modifications may be made. Forexample, other useful implementations could be achieved if steps of thedisclosed techniques were performed in a different order and/or ifcomponents in the disclosed systems were combined in a different mannerand/or replaced or supplemented by other components. Accordingly, otherimplementations are within the scope of the disclosure.

What is claimed is:
 1. A force-sensitive capacitive sensor comprising: afirst conductive plate; a second conductive plate that is spaced apartfrom the first conductive plate; a compressible dielectric insulatorpositioned between the first conductive plate and the second conductiveplate; a protective insulator that encases the first conductive plate,the second conductive plate, and the compressible dielectric insulator;and a circuit attached to the first conductive plate and the secondconductive plate, the circuit being configured to: sense a change incapacitance between the first conductive plate and the second conductiveplate caused by compression of the compressible dielectric insulatorresulting from a person occupying a support surface under which theforce-sensitive capacitive sensor is placed, based on the sensed changein capacitance between the first conductive plate and the secondconductive plate, detect a binary occupancy state of the support surfaceby detecting that the sensed change in capacitance has crossed anactivation threshold set using at least one acquired signal in anunoccupied state, and transmit output indicating the detected binaryoccupancy state, wherein the circuit is configured to detect the binaryoccupancy state of the support surface by: determining a representativeacquired signal in an unoccupied state based on values of measuredcapacitance between the first conductive plate and the second conductiveplate in the unoccupied state; setting the activation threshold based onthe representative acquired signal in the unoccupied state and a valuethat defines an amount of signal change required for activation; aftersetting the activation threshold, determining that a measuredcapacitance between the first conductive plate and the second conductiveplate has crossed the activation threshold; and based on thedetermination that the measured capacitance between the first conductiveplate and the second conductive plate has crossed the activationthreshold, detecting occupancy of the support surface.
 2. Theforce-sensitive capacitive sensor of claim 1, wherein the circuit isconfigured to measure capacitance between the first conductive plate andthe second conductive plate and transmit the measured capacitancebetween the first conductive plate and the second conductive plate. 3.The force-sensitive capacitive sensor of claim 1, wherein the circuit isconfigured to measure capacitance between the first conductive plate andthe second conductive plate, translate the measured capacitance to forceapplied to the force-sensitive capacitive sensor, and transmit the forceapplied to the force-sensitive capacitive sensor.
 4. The force-sensitivecapacitive sensor of claim 1, wherein the circuit is configured todetect the binary occupancy state of the support surface by detectingoccupancy based on measuring a force greater than the activationthreshold.
 5. The force-sensitive capacitive sensor of claim 1, whereinthe circuit is configured to: determine that the measured capacitancebetween the first conductive plate and the second conductive plate hascrossed the activation threshold by determining that the measuredcapacitance between the first conductive plate and the second conductiveplate has crossed the activation threshold for a pre-defined number ofsamples; and detect occupancy of the support surface based on thedetermination that the measured capacitance between the first conductiveplate and the second conductive plate has crossed the activationthreshold for the pre-defined number of samples.
 6. The force-sensitivecapacitive sensor of claim 1, wherein setting the activation thresholdbased on the representative acquired signal in the unoccupied state andthe value that defines the amount of signal change required foractivation comprises setting the activation threshold as therepresentative acquired signal in the unoccupied state minus the valuethat defines the amount of signal change required for activation.
 7. Theforce-sensitive capacitive sensor of claim 1, wherein the circuit isfurther configured to detect a lack of occupancy of the support surfaceand output the detected lack of occupancy.
 8. The force-sensitivecapacitive sensor of claim 1, wherein the circuit is configured tocalibrate the force-sensitive capacitive sensor by determining that thesupport surface is unoccupied based on user input, measuring capacitancein the unoccupied state, and using the measured capacitance as abaseline measurement for calibrating the force-sensitive capacitivesensor.
 9. The force-sensitive capacitive sensor of claim 1, wherein thecircuit comprises a wireless radio configured to transmit the outputwirelessly to a remote location.
 10. The force-sensitive capacitivesensor of claim 1, wherein the circuit includes electronic storageconfigured to store data based on the sensed change in capacitancebetween the first conductive plate and the second conductive plate andthe circuit is configured to transmit the output based on the datastored in the electronic storage.
 11. The force-sensitive capacitivesensor of claim 1, wherein the first conductive plate, the secondconductive plate, the compressible dielectric insulator, and theprotective insulator are all flexible such that the force-sensitivecapacitive sensor is flexible.
 12. The force-sensitive capacitive sensorof claim 1, wherein the first conductive plate and the second conductiveplate are constructed of metalized foil.
 13. The force-sensitivecapacitive sensor of claim 12, wherein with the first conductive plateand the second conductive plate include plastic backing to enhancerigidity of the metalized foil.
 14. The force-sensitive capacitivesensor of claim 1, wherein the compressible dielectric insulator isconstructed of a medium density, non-conductive foam.
 15. Theforce-sensitive capacitive sensor of claim 1, wherein the protectiveinsulator comprises a first protective insulator and a second protectiveinsulator sealed to the first protective insulator to encase the firstconductive plate, the second conductive plate, and the compressibledielectric insulator.
 16. A force-sensitive capacitive sensorcomprising: a first conductive plate; a second conductive plate that isspaced apart from the first conductive plate; a compressible dielectricinsulator positioned between the first conductive plate and the secondconductive plate; a protective insulator that encases the firstconductive plate, the second conductive plate, and the compressibledielectric insulator; and a circuit attached to the first conductiveplate and the second conductive plate, the circuit being configured to:sense a change in capacitance between the first conductive plate and thesecond conductive plate caused by compression of the compressibledielectric insulator resulting from a person occupying a support surfaceunder which the force-sensitive capacitive sensor is placed, based onthe sensed change in capacitance between the first conductive plate andthe second conductive plate, detect a binary occupancy state of thesupport surface, and transmit output indicating the detected binaryoccupancy state, wherein the circuit is configured to detect a lack ofoccupancy of the support surface and output the detected lack ofoccupancy, and wherein the circuit is configured to detect the lack ofoccupancy of the support surface by: determining a representativeacquired signal in an occupied state based on values of measuredcapacitance between the first conductive plate and the second conductiveplate in the occupied state; setting a deactivation threshold based onthe representative acquired signal in the occupied state; after settingthe deactivation threshold, determining that a measured capacitancebetween the first conductive plate and the second conductive plate hascrossed the deactivation threshold; and based on the determination thatthe measured capacitance between the first conductive plate and thesecond conductive plate has crossed the deactivation threshold,detecting a lack of occupancy of the support surface.
 17. Theforce-sensitive capacitive sensor of claim 16, wherein the circuit isconfigured to: determine a representative acquired signal in anunoccupied state based on values of measured capacitance between thefirst conductive plate and the second conductive plate in the unoccupiedstate; and set the deactivation threshold based on the representativeacquired signal in the occupied state and a difference between therepresentative acquired signal in the unoccupied state and therepresentative acquired signal in the occupied state.
 18. Theforce-sensitive capacitive sensor of claim 17, wherein the circuit isconfigured to set the deactivation threshold based on the representativeacquired signal in the occupied state and the difference between therepresentative acquired signal in the unoccupied state and therepresentative acquired signal in the occupied state by setting thedeactivation threshold as the representative acquired signal in theoccupied state plus the difference between the representative acquiredsignal in the unoccupied state and the representative acquired signal inthe occupied state multiplied by a proportion expressed as a numberbetween 0 and
 1. 19. The force-sensitive capacitive sensor of claim 16,wherein the circuit is configured to: determine that the measuredcapacitance between the first conductive plate and the second conductiveplate has crossed the deactivation threshold by determining that themeasured capacitance between the first conductive plate and the secondconductive plate has crossed the deactivation threshold for apre-defined number of samples; and detect the lack of occupancy of thesupport surface based on the determination that the measured capacitancebetween the first conductive plate and the second conductive plate hascrossed the deactivation threshold for the pre-defined number ofsamples.